I've used a vibratory trencher. (If you google you will find plenty of them.) The nice thing with the vibratory trencher is that very little damage is done to the sod, and very little patching / reseeding is needed.

The vibratory trencher basically uses a knife to slit the soil, and there is a feed tube that feeds wire or flexible pipe/conduit into a sleeve in the knife that exits at the bottom of the knife. In Michigan the soil in the area I used one was quite sandy, but the machine would bury the pipe 4' deep! (Otherwise you get to winterize the sprinkler system every year.) In Houston with very dense clay soil I used one to put a system about 18" deep, in very heavy gumbo/clay soil.

I would suggest the vibratory trencher if you can find one at a rental place in your area. Another option is to ask the sprinkler system install guys, as many will use these to lay pipe for sprinkler systems.